Acoustic emission is the phenomenon of transient elastic-wave (acoustic or ultrasonic) generation due to a rapid release of strain energy caused by a structural alteration, such as fracture or plastic distortion, in a solid material. Distortion or precursory indications of fracturing in a material are detected by measuring elastic waves (referred to hereinafter as acoustic-emission waves) which are generated due to acoustic emission as a load is applied to the material. This acoustic-emission method is in practical application in fatigue tests and materials research.
An ultrasonic receiver element made of a piezoelectric element is used in an acoustic-emission sensor which detects acoustic-emission waves. A balanced type acoustic-emission sensor is disclosed in the Japanese Laid-Open Publication No. 20890 (1976) (Tokukaisho 51-20890). In the balanced type acoustic-emission sensor, a contacting area of the piezoelectric element with a test material is provided with electrical insulating material and two piezoelectric elements are arranged in a two-step configuration. This results in a reduction in electrical noise picked up by the sensor and a reduced occurrence of phase differences between the acoustic-emission waves detected by each of the piezoelectric elements.
When ultrasonic waves are propagated through a material, longitudinal waves are transmitted faster than transverse waves. Accordingly, if only the longitudinal waves are detected from among the acoustic-emission waves, a magnitude of acoustic emission from an acoustic-emission wave generating source can be detected accurately, without any interference from the transverse waves which reach the acoustic-emission sensor after the longitudinal waves.
However, with the conventional arrangement, both the longitudinal waves and the transverse waves are detected. Consequently, problems exist such as inaccurate detection of distortion or of precursory indications of fracturing in a material.